At present, laser systems for projecting images to an audience still occupy niche markets in planetariums, nightclubs, and special venues (Niagara Falls, for example), although pocket laser projectors for use with laptop computers and cell-phones are now emerging. Generally, widespread adoption of laser projection has been delayed by the lack of compact, low-cost, high-power RGB lasers. However, issues related to eye safety have also lingered in the background, preventing wider adoption of existing technologies.
Many laser systems, projectors included, provide safety interlocks that shut down the lasers when the system enclosure is compromised. Other systems provide pro-active safety controls that shut the system down when internal components fail. For example, U.S. Pat. No. 4,613,201 to Shortle et al. describes a scanning laser projection system that shutters the laser beams when malfunctions of the scanner (a micro-mirror, AOM, or galvo) are detected. As another example, U.S. Pat. No. 6,661,820 to Camilleri et al., uses internal controls to provide eye safe laser illumination for a stop-action laser-illuminated photography system. In particular, Camilleri '820 provides a fault detection scheme to monitor the operation of a laser projection system relative to internal electronic or software failures that could cause the system to operate unsafely. Camilleri '820 also includes a laser control system, that allows peak laser power, average laser power, and laser pulse parameters to be controlled, relative to a predetermined and appropriate safety standard that has been provided for a given application.
Laser safety can also be managed for beams traveling in free space outside an enclosure. As a first example, U.S. Pat. No. 4,884,275 to Simms provides for a portable IR laser illuminator that includes a series of infrared light responsive photoelectric detectors arrayed around the exit aperture of the light source. These detectors can receive a portion of any light reflected from the laser light source by an object intruding into the laser beam path. The detectors are intended to be particularly responsive to reflected light directed from within a danger zone of a few inches from the exit aperture. When incident light corresponding to the frequency of the laser light source is detected, a safety switch turns off the power supply to the laser light source
In the case of front-side projection systems, efforts have been made to manage at-risk situations related to eye safety in the viewing environment. In particular, projection systems that provide active detection of intruding objects or people in the beam path beyond the projector enclosure (or housing), and then respond with corrective actions to reduce or re-direct the light output have been described. As an example, U.S. Pat. No. 6,361,173 to Vlahos et al., describes an image projection system that includes an IR light source and an IR sensitive imaging camera that are both directed towards the screen. When a subject (person) enters the projection beam, the subject's infrared reflection is likely to be higher or lower than the uniform infrared luminance level of the projection screen. The change in the IR reflection signal is correlated with projected image pixels. Then, using a digital matte, video program signals to the pixel addresses associated with the subject location are changed so as to reduce the corresponding light exposure. While Vlahos '173 is motivated to reduce the annoyance of the subject (a presenter addressing an audience, for example) to being blinded by the projection light, rather than by eye safety concerns, the controlled inhibition (<5% full intensity) of the projection light on the detected subject provides much the same effect.
An associated patent, U.S. Pat. No. 6,789,903 to Parker et al., improves upon Vlahos '173 by using structured or patterned IR illumination. Structured or patterned IR illumination is useful in locating an obstruction, and also in isolating patterns from the ambient illumination or patterned clothing worn by the presenter, that can otherwise confuse interpretation of the detected IR signals. An imaged pattern indicative of an obstruction can be compared to a reference frame pattern to generate differential signals that locate the obstruction. Again, mattes, which represent modified portions of an image, can be generated for pixels associated with an obstruction, thereby preventing illumination of the obstruction. Another related patent, U.S. Pat. No. 6,796,656, to Dadourian et al., uses a similar method, except that the screen is patterned with high gain retro-reflective beads that direct bright IR light to the IR camera, thereby providing high signal levels useful in generating mattes to inhibit portions of the projected image.
In a similar fashion to Parker '903, U.S. Pat. No. 7,210,786 to Tamura, et al., compares a patterned IR light reference image taken of the image projection area (without obstructions) to images captured during system operation with the same patterned IR light. In particular, image comparison and object detection algorithms locate image differences, thereby locating obstructions. Image data is then blanked using oversized image masks, providing eye safety with margin for potential motion of the person.
Front projection laser systems with perimeter monitoring to anticipate incoming people or object have also been described. As one example, U.S. Pat. No. 6,002,505 by Kraenert et al., uses an IR laser to scan beyond the extent of the projected visible image. IR sensors can detect changes in light reflection caused by intruding objects, resulting in the safety circuit triggering system operation into a safer mode of operation. Likewise, U.S. Pat. No. 6,575,581 to Tsurushima projects IR light onto the screen including to a perimeter area larger than the projected image area. Reflected light from the screen and from any incoming obstacles is detected and analyzed, thereby identifying and locating any changes (reductions) in the reflected light intensity caused by the obstruction.
By comparison, U.S. Pat. No. 7,144,117 to Kojima et al., describes a scanning laser projection system which scans an IR detection beam within the image projection area, but in advance of the scanned image light. In particular, Kojima '117 describes a scanning laser projector in which color images are created using color channels comprising an appropriate laser (R, G, or B) and an associated grating light valve (GLV) modulator array. The color beams are combined and scanned across the screen using a rotating scan mirror. In parallel, a second IR laser beam source is scanned across the screen by the same scan mirror, but in advance of the image light. An IR sensitive camera can then detect intruding objects as changes in the reflected light signal. When an object is detected, control signals are generated to reduce the light intensity of the RGB lasers that provide the image light to (human) eye safe levels.
In another method, commonly assigned U.S. Pat. No. 6,984,039 to J. Agostinelli describes a scanning laser projection system that has a camera that captures screen images, nominally of the image provided by the projector itself. The input image, which is known, is compared to the projected image captured by the camera. Image analysis can then find differences indicative of the presence of an obstruction in the projection space near the screen. The processor then determines modifications to the projection image data that can be applied, such that scan lines can be modified, or “blanked”, so that no light is projected over the obstructed area. As Agostinelli '039 intends to provide conformal blanking around the object, the image differencing analysis essentially provides silhouettes or outlines of the obstructing object. The silhouette positions can be tracked in time as the object moves, so that the image blanking can be provided in the direction of object movement, while image content can again be projected into the vacated space. This is known as image filling.
Agostinelli '039 also teaches that the camera can be offset from the projection optical axis, so that the shadow cast by the object, relative to the projected light, can be detected. As the shadow contrast is high relative to the projected light, large image differences are provided, making object detection easier. Additionally, Agostinelli '039 teaches that facial feature detection, eye location, and red-eye detection algorithms can all be used to locate eye regions, so that projected light can be specifically blocked in the eye region areas.
While the prior art discussed above describes laser safety features in various ways, it does not distinguish between safety needs near the projector, compared to those proximate to the screen. In particular, close to the projector, where energy levels are high and likely in excess of published safety standards for exposure, complete blanking of the projected light given the presence of an obstruction is warranted. Indeed, highly robust detection mechanisms with miniscule failure rates are required. Even though the obstructing object itself may not be a face and eye, a highly reflective surface in the beam path could direct an intense beam into an eye that is otherwise located outside of the projection light path. Thus, the detection means need to be universally successful, regardless of the nature of the object.
On the other hand, at locations proximate to the screen, energy levels typically are actually reduced to safer levels. For example, in the space immediately in front of the screen, the laser safety levels of a projector may be Class 3R (IEC Laser Classification), which is considered marginally unsafe for intra-beam viewing. In a Class 3R area, it is assumed that an observer can react to eye exposure via the eye aversion response, thereby avoiding damage. This suggests that in the near screen region, where energy levels are nominally safe, safety screening could be subtler than has been suggested. Notably, in the laser safety prior art discussed above, positive detection of objects near the screen typically results in large portions of the image being blanked. If the object is innocuous and inanimate, such blanking may irritate viewers. Thus, by comparison, it can be desirable to discriminate between animate (alive) and inanimate (not-alive) objects, and in the latter case, allow projection to continue without interruption.
A more finessed or subtle screening can also be directed to eye-based screening methods, using eye detection and tracking algorithms, for example. However, careful consideration is required for such approaches. It is observed that the eye safety standards are generally written anticipating that potential eye damage will be reduced in part by the eye aversion response to intense light. However, human (or animal) behaviors, common in a residential environment, may lead to potentially unsafe situations not fully anticipated by the standards. As an example, if an animate being deliberately looks back into the projector, over-riding the eye aversion response, eye damage may still occur. As another example, eye safety standards may not also fully account for sudden eye exposure to laser light of a (scotopically or mesopically) dark-adapted individual, whose eyes have heightened bio-chemical sensitivity as well as large pupil dilation. Additionally, in a residential environment, the exposed animate being can easily be an animal, such as a common pet like a cat or a dog. Cat eyes, as an example, have ˜6× greater light sensitivity than human eyes. However, the laser safety standards are human centric, and do not take animal eyes into account. Thus, while current laser safety standards may not address such situations, eye damage may potentially occur near the screen, despite that area being rated as nominally safe.
Given these concerns, there are legitimate reasons to specifically target projection safety screening and projection image blanking towards animate objects generally, and then particularly to the likely head or facial regions thereon. This is true whether entire lines of image data are blanked or the image blanking is more conformal to the body or head region of the object. In general, robust eye safety management needs greater discrimination than taught in the prior art. Face detection algorithms, red-eye detection algorithms, or body shape detection algorithms can all be insufficient. The potential of complex body shapes, obscuration by clothing, the presence of animals, and abrupt motion, all complicate eye safety management. In practice, robust eye safety management should emphasize detection of animate objects, detection of candidate head regions using multiple methods, and prioritization of projection light blanking to animate objects and candidate head regions. As the prior art has neglected these issues, and has not developed solutions thereto, opportunities remain for improved eye safety management with regards to image projection systems, including laser projection systems.